Matheny cut himself with the knife, a birthday gift he had just opened, and was in surgery as shocked teammates learned the news in a team meeting about 3 1/2 hours before a game against Cincinnati.
Matheny was putting the knife in its sheath and it went right through, severing two flexor tendons and a nerve, Cardinals spokesman Brian Bartow said. The team anticipates he'll make a full recovery for next season.
"I feel worse for him than I do for us," manager Tony La Russa said.
The loss could be a serious blow to the NL Central champions. Matheny, a contender for a Gold Glove, threw out 51 percent (46-for-90) of would-be basestealers and hit .261 with 22 doubles, six homers and 47 RBIs.
"To me, he's our MVP," closer Dave Veres said. "We couldn't have won without him, and I'm sure a lot of guys feel the same way."
The backups at the position are Carlos Hernandez, acquired just before the trade deadline, and Eli Marrero. Hernandez, who was on the Padres' 1998 World Series team, has been bothered by a bad back, and Marrero has played sparingly after missing about two months because of a torn ligament in his left thumb.
Marrero played Friday and La Russa said Hernandez would start on Saturday. La Russa wouldn't reveal his postseason plans for the position.
Marrero was hitting .230 in 51 games with five homers and 17 RBIs and had thrown out seven of 13 basestealers, but the Cardinals consider him weaker at calling pitches. Hernandez had thrown out six of 21 basestealers and was batting .258 with two homers and 32 RBIs in 72 games.
"The games are still going to be played and they're still going to count," La Russa said. "So we need to understand what Mike meant and get ready to play without him.
"I mean, it's a situation we've faced before, a whole bunch of times."
The team has overcome several major injuries this season. Mark McGwire was out two months with a bad knee and has been reduced to pinch hitting since his return earlier this month. Third baseman Fernando Tatis missed about two months with a torn groin, and pitcher Andy Benes was out of the rotation about a month with a knee injury that will require sugery after the season.
"We've just got to persevere," pitcher Pat Hentgen said. "We lost Tatis, we lost McGwire and here we are with 94 wins. We've gotten this far because we have 25 good players that can play the game, and I think we'll continue to win."
Matheny's injury continues a Cardinals tradition of losing key players right around the postseason.
In 1996, Ray Lankford tore the rotator cuff in his left shoulder making a diving catch on the second-to-last day of the regular season, and pitcher Donovan Osborne cut his pitching hand when he pulled a broken bottle of champagne out of a box during the clinching celebration.
In 1987, they made it to the World Series without Jack Clark (sprained ankle) and Terry Pendleton (rib cage), and in 1985 Vince Coleman was injured by a mechanical tarp.
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